1. Field of the Invention
The present invention relates to a film bulk acoustic resonator (hereinafter, referred to as an FBAR), and an FBAR based device, such as a duplexer, and more particularly to an FBAR, an FBAR based device, and a manufacturing method thereof, which can achieve miniaturization, and reduction of a manufacturing cost and enhancement of a yield due to a simplified process.
2. Description of the Related Art
In recent years, wireless communication devices have tended to become much leaner, and enhanced and diversified in their quality and functions due to the development of the communication industry. This recent trend sincerely requires miniaturization and enhancement in quality related with various elements for use in the wireless communication devices.
In order to satisfy such requirements to miniaturization, therefore, currently, active development is targeting studies for manufacturing essential components of wireless communication devices, such as a filter and a duplexer, by using FBARs. The FBARs are preferable for integration due to their thin film shapes, and have good properties.
Typically, the FBARs are usually formed in such a fashion that a piezoelectric layer is formed on a wafer, and upper and lower electrodes are formed at the upper and lower surfaces of the piezoelectric layer, respectively, for applying electricity to the piezoelectric layer so as to induce oscillation thereof. Further, a desired air gap is formed at the lower surface of the piezoelectric layer in order to improve a resonance property of the piezoelectric layer.
FIGS. 1A and 1B are sectional views, respectively, illustrating different structures of conventional duplexer devices implemented by using the FBARs formed as stated above.
The conventional FBAR based duplexer device shown in FIG. 1A comprises at least two FBARs 12, mounted on a substrate 11 serving as a lower support, for forming a Tx (transmitter) filter and an Rx (receiver) filter, respectively. The substrate 11 is formed with a common terminal and transmission/receiving terminals, and circuit patterns for electrically connecting the terminals to the Tx and Rx filters implemented by the FBARs. After the FBARs 12 are electrically connected to the circuit patterns formed at the substrate 11, in order to achieve a complete sealing of all of the FBARs 12, a molding portion 13 is formed on the substrate 11 by the use of certain sealing material.
As the substrate 11, due to the complexity of implemented circuits thereof, printed circuit board (PCB) type or low temperature co-fired ceramic (LTCC) type substrates are mainly used, but the PCB substrates are more preferable since they have many advantages, such as a low price, good properties and high productivity. In case of using the PCB substrates, as shown in FIG. 1A, it is necessary to provide certain protective structures on the FBARs 12, respectively, for protecting device functional portions, that is, piezoelectric layers, air gaps, and electrode layers, of the FBARs 12 from a molding process. These protective structures may be formed, for example, by processing a wafer having a certain thickness according to a wafer level package (WLP) technique, and bonding the processed wafer onto a substrate wafer of a corresponding FBAR.
In case that the FBARs 12 are formed with the protective structures, respetively, as stated above, however, the overall structure and manufacturing process of the FBARs 12 are disadvantageously complex, since the protective structures should be configured so as to be electrically connected to the device functional portions inside the FBARs 12, respectively, as well as protect the device functional portions.
When an FBAR is bonded to a substrate obtained by a wafer level package process, the obtained wafer level package type FBAR device has a very small size of about 1 millimeter in length and width. Due to the small size, a cap and substrate constituting the package has a sealing area corresponding to only about 30 to 100 square micrometers, with the exception of a driving portion. Since the FBAR device can endure only about 30° C. during its bonding process, there is a considerable restriction in a sealing method for securing a good reliability.
Even when a large amount of the FBAR devices are produced through any precision processes, due to a complexity in process thereof, it is difficult to obtain an appropriate yield.
In case that an LTCC technique is adopted in order to eliminate the above problems, as shown in FIG. 1B, first, a plurality of ceramic sheets are vertically laminated so as to form an LTCC substrate 15, which is defined therein with a cavity, and then a plurality of FBARs 16 are mounted inside the cavity defined in the LTCC substrate 15. After electrically connecting the FBARs 16 to the substrate 15 by bonding wires therebetween, a metal lead 17 is fused or seam-sealed on the LTCC substrate 15 above the FBARs 16.
In this case, since the LTCC substrate 15 is configured in such a fashion that circuits having a duplexing function are arranged therein in a multi-stage form, it is possible to achieve a reduction in size, compared with the case of using a single layer PCB substrate. Further, the LTCC substrate does not need a molding process. Furthermore, according to the structure of the LTCC substrate 15 as stated above, the LTCC substrate 15 already owns certain protective structures for protecting the FBARs 16. The protective structures are side walls obtained by defining the cavity in the substrate 15. Therefore, the LTC substrate 15 does not need separate protective structures. That is, the FBARs 16 only comprise an air gap, piezoelectric layer and electrode layers vertically arranged in series on an FBAR substrate wafer.
The LTCC technique, however, causes torsion of the LTCC substrate 15 during a LTCC firing process, resulting in a serious leak problem due to inferior bonding between the lead 17 and the LTCC substrate 15. Further, due to the fact that the LTCC substrate 15 is formed by vertically laminating a plurality of the ceramic sheets, there is a high possibility of producing any defects in the LTCC substrate itself.
Although the above techniques have been achieved according to a most effective method for miniaturization, since a possibility of inferiority due to complex processes always exists, it is difficult to secure a profit margin required for mass production, causing an unnecessarily high manufacturing cost, and to increase a possibility of producing inferior products due to an operator's carelessness.